Is the [CH3CN]/[CH3OH] abundance ratio in comets inherited from the early stages of planet formation?

Comets are considered fossils from the early solar system, and comparing their composition to that found in planet-forming disks surrounding young solar analogues (10⁴ — 10⁶ years old) can provide key insights into the physical, chemical, and evolutionary processes that shaped our planetary system [1,2]. 

Our previous analysis of the [CH₃OH]/[H₂CO] and [CH₃OH]/[NH₃] abundance ratios in a database of 35+ comets from infrared and sub-mm high-resolution ground-based spectroscopy, showed a consistency between Class 0 hot-corinos, inner regions of Class II disks, and comets, supporting the so called “inheritance scenario” [3]. The study also underlined the importance of including in the analysis as many objects as possible, in order to identify possible data biases and avoid consequent misinterpretation of the results. 

Here we will present how the same statistical methodology can be applied to additional species, investigating in detail the [CH₃CN]/[CH₃OH] abundance ratio. Methyl cyanide is one of the most abundant and widely diffuse interstellar complex organic molecules, which most likely form in the gas-phase from methanol (see for example [4]), and as methanol it may play an important role in the synthesis of more complex organics as well as prebiotic molecules. We will investigate statistically the [CH₃CN]/[CH₃OH] abundance ratios in comets, considering a large sample of data from the literature (e.g., [7,8]) and comprising recent results from ALMA and CRIRES⁺ observations of comets (e.g., C/2023 A3 (Tsuchinshan-ATLAS) and 12P/Pons-Brooks). When tested against the latest findings in star-formation research (e.g., [9,10]), the [CH₃CN]/[CH₃OH] ratios in comets and hot-corinos are consistent, pointing again in favour of an inheritance scenario, as we previously found for the [CH₃OH]/[H₂CO] and [CH₃OH]/[NH₃] ratios. 

References: [1] Mumma, M. J., & Charnley, S. B., 2011, ARA&A, 49, 47; [2] Ceccarelli, C., et al., 2023, in Protostars and Planets VII: [3] Lippi, M., et al., 2024, ApJ, 970, L5; [4] Giani, L., et al., 2023, MNRSA, 526, 3, 4535;  [7] M. A., Cordiner, et al., 2023, ApJ, 953, 59; [8] Biver, N., et al., 2022, A&A 668, A171; [9] Belloche, A., et al. 2020, A&A, 635, 198, [10] Yang et al., 2021, APJ, 910, 1.